The indicator-dilution method of measuring blood flow is based on the
premise that, when an indicator substance is added to circulating blood,
the rate of blood flow is inversely proportional to the change in concen-
tration of the indicator over time. If the indicator is a temperature, the
method is known as
thermodilution.
The thermodilution method is illustrated in Figure 8.5. A dextrose or
saline solution that is colder than blood is injected through the proximal
port of the catheter in the right atrium. The cold fluid mixes with blood
in the right heart chambers, and the cooled blood is ejected into the pul-
monary artery and flows past the thermistor on the distal end of the
catheter. The thermistor records the change in blood temperature with
time; the area under this curve is inversely proportional to the flow rate
in the pulmonary artery, which is equivalent to the cardiac output in the
absence of intracardiac shunts. Electronic monitors integrate the area
under the temperature–time curves and provide a digital display of the
calculated cardiac output.
Thermodilution Curves
Examples of thermodilution curves are shown in Figure 8.6. The low car-
diac output curve (upper panel) has a gradual rise and fall, whereas the
high output curve (middle panel) has a rapid rise, an abbreviated peak,
and a steep downslope. Note that the area under the low cardiac output
curve is greater than the area under the high output curve (i.e., the area
under the curves is inversely related to the flow rate).
Sources of Error
Serial measurements are recommended for each cardiac output determi-
nation. Three measurements are sufficient if they differ by 10% or less, and
the cardiac output is taken as the average of all measurements. Serial
measurements that differ by more than 10% are considered unreliable (14).
Variability
Thermodilution cardiac output can vary by as much as 10% without any
apparent change in the clinical condition of the patient (15). Therefore, a
change in thermodilution cardiac output should exceed 10% to be consid-
ered clinically significant.
Tricuspid Regurgitation
Regurgitant flow across the tricuspid valve can be common during posi-
tive-pressure mechanical ventilation. The regurgitant flow causes the
indicator fluid to be recycled, producing a prolonged, low-amplitude
thermodilution curve similar to the one in the bottom frame of Figure 8.6.
This results in a falsely low cardiac output measurement (16).
Intracardiac Shunts
Intracardiac shunts produce falsely high thermodilution cardiac output
measurements. In right-to-left shunts, a portion of the cold indicator
142
Hemodynamic Monitoring
